Familial Hemophagocytic Lymphohistiocytosis (FHLH) is almost universally fatal unless aggressively treated soon after diagnosis, and corrected with allogeneic bone marrow transplantation. All forms of FHLH likely result from genetic defects in the natural down regulating mechanisms of immune/inflammatory responses. Three autosomal recessive gene defects underlie 40-50% of primary (familial) cases worldwide: perforin (20-30%), the major immune cytotoxic protein, MUNC 13-4 (20%), a protein involved in exocytosis of perforin-bearing cytotoxic granules during apoptosis and STX11, member of soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNARE). Thus in more than half of the FHLH cases, the affected individuals have other as yet unknown genetic alterations. The classic genetic approach to identify these genes, through linkage analysis, has limitations because linkage analysis requires DNA from relatively large numbers of affected and unaffected members of a single family. Such families are unusual in North America. We estimate at least six or seven genes could be responsible to the various genetic forms of FHLH. In this study we propose novel procedures, to search for candidate genes responsible for the remaining 50-60% of FHLH cases. It is estimated that 30% of inherited genetic disorders in humans result from premature termination codon mutations. Nonsense mutations in mRNA activate the nonsense-mediated decay (NMD) pathway, which results in the degradation the mutant mRNA. We will identify gene expression profiles that will distinguish FHLH2, FHLH3 and FHLH4 from other genetic forms of FHLH. We propose that NMD pathway could be utilized to identify FHLH candidate genes through expression microarray hybridization. In addition, candidate genes could be identified through inhibition of the nonsense-mediated decay (NMD) mechanism in HVS immortalized CD8+T cells from patients with FHLH of yet unknown genetic cause. Alternatively, we will use RNAinterference strategy to identify candidate genes in lymphohistiocytosis susceptibility regions of chromosome 9q21.3-22 and 11q25 and among genes coding for SNARE proteins essential for direct, controlled and very rapid fusion of phospholipids membranes.